From: R.J.Neill Craven (ncraven@xxxxxxxxxxxxxxx)
Date: Wed Nov 14 2001 - 15:50:04 GMT-3
Unfortunately the referenced URL does not explain the time
relationship between Bc and CIR. Have a look at "IP Quality of
Service", Srinivas Vegesna, Cisco Press, 2001. Although I don't agree
with the definition of conformed burst on page 43 (I think it should
read "This is the amount of traffic that is allowed to ENTER the
token bucket ...") I find the discussion to be quite good.
As for rate limiting versus traffic shaping, I believe they both make
use of the same tools but have different contexts. Using my earlier
analogy to salary, rate limiting controls the amount of money that is
made available (how much is put in the bank), whereas traffic shaping
controls the way the money is spent (how it is removed from the
bank). For example, when I was at school I received an allowance of
$4 per week. (Yes, that was many years ago!) How I chose to spend my
allowance was independent of the rate at which I received my
allowance. The only absolute rule was that I couldn't spend my
allowance faster than I received it.
In the realm of switching, I think of a bucket with two spigots, one
allowing the bucket to fill (or accept traffic to be processed) by up
to Bc bytes every Tc seconds (rate limiting), and a second, defining
the rate at which the bucket is emptied (traffic shaping), again
defined by a CIR, Bc, and Tc. The former can be quite coarse (lumpy?)
in that the arrival of data in the bucket can be quite bursty,
whereas the delivery of the data to the network might need to be
quite smooth. In either case, though, we define the desired rate
(CIR) and establish the bursty nature of the traffic by manipulating
the Bc to obtain the desired Tc. The smaller the Tc, the lower the
tolerance to burstiness and visa versa.
Cheers,
Neill
At 11:24 AM -0600 14/11/01, Wade Edwards wrote:
>I think you are thinking of CAR like GTS or FRTS which are traffic
>shaping technologies. This is the document that I have always used with
>CAR.
>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios111/cc111/ca
>r.htm
>
>It gives some examples and states that CAR propagates bursts. It does
>no smoothing or shaping of traffic as you state in your email. BTW I
>really liked your explaination.
>
>Give it a look and let me know.
>
>L8r.
>
>-----Original Message-----
>From: R.J.Neill Craven [mailto:ncraven@cravenworks.com]
>Sent: Wednesday, November 14, 2001 11:01 AM
>To: Wade Edwards
>Cc: tom cheung; kevin@btamail.net.cn; ccielab@groupstudy.com
>Subject: RE: How to calculate CAR?
>
>
>Wade,
>
>I have very different view as to how CAR works! The first number
>(1544000) is indeed the bit rate, but the remaining two values are
>NOT in addition to the first. The second, the burst (in bytes) and
>sometimes referred to as Bc, helps to indicate the size of the token
>bucket and is used to determine the time over which the traffic is
>rate limited (Tc). In particular, Tc=Bc*8/CIR. The "*8" is used to
>convert the bytes of Bc to bits.
>
>Tc is a very important parameter in rate limiting or traffic shaping.
>Suppose as a new CCIE you are offered a salary of $120,000 per year
>but you are offered several payment strategies. You could take one
>payment of $120,000, or 12 payments of $10,000, or 24 payments of
>$5,000, or .... I have never been very good at budgeting money so I
>would choose to spread the salary over more periods. In other words,
>I want the salary to arrive at a fairly even rate. Someone else might
>prefer the lump sum approach. Any way you look at it though, we are
>only getting $120,000 per year, each! In the context of CAR (and
>disregarding the bits and bytes business) I would define the rate
>limit for my salary as "rate-limit input $120000 $5000 $5000". Tc
>would be 5000/120000 or 1/24th of a year giving me 24 payments of
>$5000 each.
>
>I think you will agree that having some way to define the time over
>which the rate is limited is particularly useful. We just need to be
>clever about how we choose the Bc to achieve the right Tc.
>
>The third parameter (typically called Be), is the excess burst and in
>the context of of my pay, indicates the maximum I can be paid during
>each pay cycle. Perhaps I have an overtime provision in my contract
>which allows me to accumulate additional pay credits. But the company
>can limit their exposure by indicating that they will not pay more
>than a certain total amount during each period. And they will not let
>me work overtime continuously, either. So if I try to inflate my pay
>package by working a lot of overtime, they are going to stop me
>working overtime, period! Suppose I am allowed to accumulate $1,000
>in overtime pay in addition to my regular salary, I would define this
>as "rate-limit input $120000 $5000 $6000".
>
>So, Kevin, to get your desired effect, I would have used "rate-limit
>input 154000 6000 8000" or something similar to allow for bursts of
>1/3rd of the average rate. (Adding 1/3rd to 6000 bytes makes 8000
>bytes; 8000 bytes during the same period increases the rate by 1/3rd
>or approximately the difference between 1.544 Mbps and 2.048 Mbps.)
>You could use bigger values than 6000 and 8000, so long as you keep
>the ratio of 3 to 4.
>
>Cheers,
>Neill
>
>At 9:33 AM -0600 14/11/01, Wade Edwards wrote:
>>The rate-limit command you specified is going to allow 1.544Mb normal
>>traffic then allow another 2.048Mb of burst traffic then it will drop
>>all traffic after 3.592Mb. The numbers should be 1544000 63000 63000.
>>This will give you 1.544Mb normal traffic then allow another .504Mb
>>(63000 bytes x 8 gives you 504000 bits or .504Mb) for a total of
>2.048Mb
>>of traffic then all traffic will be dropped.
>>
>>The burst numbers are in addition to the normal traffic and the second
>>and third numbers are in bytes and the first number is in bits. Hope
>>this helps.
>>
>>L8r.
>>
>>-----Original Message-----
>>From: tom cheung [mailto:tkc9789@hotmail.com]
>>Sent: Tuesday, November 13, 2001 8:03 PM
>>To: kevin@btamail.net.cn; ccielab@groupstudy.com
>>Subject: Re: How to calculate CAR?
>>
>>
>>rate-limit in 1544000 256000 256000 conform transmit exceed drop
>>
>>
>>>From: Kevin <kevin@btamail.net.cn>
>>>Reply-To: Kevin <kevin@btamail.net.cn>
>>>To: "ccielab@groupstudy.com" <ccielab@groupstudy.com>
>>>Subject: How to calculate CAR?
>>>Date: Wed, 14 Nov 2001 9:18:52 +0800
>>>
>>>Hello all, I do learn Commit Access Rate(CAR).
>>>I found there are three parameter about car.
>>>If I want to give one my custom a contract bandwidth on FastEthernet
>>which
>>>commit access rate 1.544M and burst size is 2.048M. How to setup the
>>second
>>>and third parameter on "rate-limit" command. Of course the first
>>parameter
>>>is 1544000, but which value is precision for normal burst size and max
>>>burst size.
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